1. Field of the Invention
The present invention relates to a lining material for glass melting furnaces, which is composed mainly of platinum or platinum alloy, a process for purifying the lining material, a glass melting furnace lined with the lining material, and a process for producing glass products.
2. Description of the Related Art
Glass is defective if it contain stones, cords, and bubbles. Stones are crystalline bodies of minerals, and they adversely affect the transparency and strength of glass. Cords are streaky amorphous bodies, and they adversely affect the refractive index of glass, distorting the image passing through glass. Bubbles also adversely affect the transparency of glass. Particularly, bubble wreckage on the surface of an LCD (liquid crystal display) substrate causes open circuits. Therefore, LCD glass plates and optical lenses should be free of the above-mentioned defects.
It is known that stones result from fine powder of refractory material when glass is melted in a melting furnace lined with refractory material, that cords result from refractory material dissolved in glass, and that bubbles result from refractory material in contact with glass. One way to prevent stones and cords is by lining with platinum or platinum alloy. One way to prevent bubbles is by incorporation of a fining agent into raw glass materials.
The above-mentioned measures are successful to a certain extent. Even a few bubbles are not tolerable for electronic glass (such as LCD glass). Complete elimination of bubbles is impossible even in the case of lining with industrial platinum of high purity (99.95 wt %), with the remainder (0.05 wt %) being unavoidable impurities. Countermeasures against bubbles are indispensable for further improvement in glass quality.
Despite their thorough investigation into production facilities involved, the present inventors were unable to pinpoint the source of bubbles. They assumed that the major source of bubbles is platinum in contact with molten glass.
It was found that industrial platinum in actual use varies in residual impurities depending on its origin and production lot and on whether or not it contains recycled platinum. An example of analytical data is given below.
TABLE 1 impurities in industrial platinum Impurities Content Palladium (Pd) 5-13 ppm Rhodium (Rh) 20-40 ppm Iridium (Ir) 0-13 ppm Osmium (Os) 30-84 ppm Arsenic (As) 4-14 ppm
Palladium (Pd) is a stable element and is unlikely to cause bubbles. Rhodium (Rh) is an element added to increase the strength of platinum and is also unlikely to cause bubbles. Osmium (Os) is liable to oxidation, giving rise to an oxide which vaporizes at a low temperature. In view of this and its high content, osmium is very likely to cause bubbles. Iridium (Ir) is oxidized at 800.degree. C. and above (although not so easily as osmium) and vaporizes at 1000.degree. C. and above. However, in view of the fact that iridium has long been used as a material for glass melting crucibles, iridium is unlikely to cause bubbles. Arsenic (As) vaporizes easily or oxidizes easily to form volatile oxides. However, it readily dissolves in glass and hence is unlikely to cause bubbles.
Accordingly, the present inventors have experimentally studied the relation between the amount of osmium and the number of bubbles. It turned out that the lower the content of osmium, the less the number of bubbles. This finding led to the present invention.